The present invention relates to a vacuum booster assembly adapted for use in automobile brakes, and more particularly to a vacuum booster assembly of the type provided therein with a diaphragm piston assembly.
A conventional vacuum booster assembly of this type comprises a housing, a piston body axially movable in the housing, a diaphragm member having an inner peripheral bead coupled over the piston body and an outer peripheral bead secured to the inner wall of the housing, the diaphragm member being arranged to subdivide the interior of the housing into a negative pressure chamber and a variable pressure chamber, valve means cooperable with the piston body for selectively communicating the variable pressure chamber with the negative pressure chamber or atmospheric air, and an annular support plate attached to one face of the diaphragm member and having an inner cylindrical portion coupled with the inner cylindrical bead of the diaphragm member to fasten the same to the piston body in an air-tight manner.
In the above described booster assembly, the annular support plate is fixed only at its inner cylindrical portion to the piston body and is resiliently flexed in an axial direction when received a load caused by the difference in pressure between the negative and variable pressure chambers. On the other hand, it is desired to make the booster assembly as small in its axial width as possible and as light in weight as possible. For this reason, it is required to make the annular support plate as small in axial width as possible without causing undesired resilient flexure, and it is further required to make the annular support plate as light in weight as possible with sufficient strength against the load applied thereto.
Furthermore, in general, the housing consists of front and rear shells coupled at their peripheral rims to each other, and the outer peripheral bead of the diaphragm member is clamped in an air-tight manner at the coupled portion of the front and rear shells. FIG. 7 illustrates the coupled portion of the front and rear shells which are respectively designated by reference numerals 1 and 2. The rear shell 2 is formed at its outer peripheral rim with a radially outwardly folded annular edge 2a and an annular clamp flange 2b extending outwardly from the folded annular edge 2a. Thus, the outer peripheral bead 3a of the diaphragm member 3 is coupled within an annular groove C in the annular clamp flange 2b and clamped by engagement with the inner wall of the front shell 1. Meanwhile, the annular clamp flange 2b is coupled with an annular stepped flange 1a of the front shell 1 which is radially inwardly bent at its circumferentially spaced positions to fasten the annular clamp flange 2b in place. In such a conventional construction, the folded annular edge 2a is formed with an annular slit S which opens outwardly to allow entry of the water therein. This results in corrosion of the annular clamp flange 2b in a short period of time. It is, therefore, required to thicken the rear shell 2 in order to ensure the durability of the annular clamp flange 2b.